Might have been thinking of a quarter rib but not something I have seen before.

The notch in the barrel is the spline cut for the rear sight sleeve. After the sleeve was put on a hole was drilled from the rear then a reamer so a square end, later they dropped the reamer.

Most gunmakers filled the spline cut, some did it so well it's darn hard to see.



MP Sadly Deceased as of 2/17/2014